A related art power supply apparatus is configured to supply AC power to a heating cod for induction heating in which a workpiece placed in a magnetic field formed by the heating coil due to the AC power supplied to the heating coil is heated by current induced in the workpiece. The power supply apparatus generally converts AC power of a commercial power supply into DC power by a converter, smoothes a pulsating current of the DC power by a capacitor, and converts the smoothed DC power into AC power by an inverter to generate high frequency AC power to be supplied to the heating coil (see, e.g., JP 2009-277577A).
The inverter typically includes a plurality of pairs of series-connected power semi-conductor devices (switching devices), the pairs being connected to each other in parallel. The heating coil is connected between series connection points between the power semiconductor devices in the respective pairs so that high frequency AC power can be supplied to the heating coil by high speed switching operations of the power semiconductor devices.
The high speed switching operation of the power semiconductor device rapidly changes the current flowing into the power semiconductor device, and the current change di/dt generates surge voltage L×di/dt between opposite terminals of the power semiconductor device due to a parasitic inductance. L of a conductive path between the power semiconductor device and the capacitor serving as a voltage source. Excessive surge voltage may cause damage to the power semiconductor device. Thus, the surge voltage is .required to be suppressed. Since the current change di/dt is determined primarily by the characteristics of the power semiconductor device, the surge voltage can be suppressed by reducing the parasitic inductance L.
As a measure for reducing the parasitic inductance, a capacitor serving as a voltage source is typically provided in the vicinity of a load. However, in a large power application such as in a power supply apparatus for induction heating, a capacitor is required to have relatively large capacitance, so it is difficult to arrange the capacitor in the vicinity of the power semiconductor device (the load). This is because the size of the capacitor increases with the increase of the capacitance.